1.Field of the Invention
For measuring the relative position of two machine parts, a scale has to be secured to one of the machine parts, and a scanning unit has to be secured to the other of the machine parts movable relative to one another. During the position measurement, a measuring graduation of the scale is scanned by the scanning unit, and position-dependent scanning signals are generated.
2. Description of the Related Art
One possible way of securing a scale on a substrate is described in East German Patent Disclosure DD 229 334 A1. In it, the scale is forced against the substrate by way of pneumatic suction. For the suction, a suction conduit is provided outside the measuring graduation and is intended to force the scale, over its entire length, against the substrate with a constant contact pressure.
The problem here is that when the scale rests on the substrate, the connection can easily be interfered with by contaminants or the development of air bubbles in the space between the scale and the substrate. These problems lead to changes in length over time and location of the measuring graduation of the scale, which in turn cause errors in the position measurement. This problem occurs to an increased degree in relatively large-area scales.
In European Patent Disclosure EP 1 783 463 A1, a scale fastening by optical contact bonding elements is explained. In large-area optical contact bonding elements, the problem is that a disturbance in the contact face can detach the entire optical contact bonding element, because the detachment spreads. To prevent this large-area detachment of the optical contact bonding connection, the contact face of EP 1 783 463 A1 is subdivided into many small contact faces spaced apart from one another. Although this embodiment does ensure a secure fastening of the scale to a substrate, nevertheless a stress-free contact pressure that is homogeneous over the entire measuring graduation plane is only poorly attained in this way.
In EP 1 783 463 A1, it is pointed out that the optical contact bonding element can be initiated by pneumatic suction of the scale against the substrate. Holding the scale on the substrate by pneumatic suction in the measurement mode of the position measuring instrument is not mentioned. Nor are any structural provisions for the pneumatic suction mentioned.
It is an object of the present invention to disclose an assembly with a scale secured to a substrate, in which the scale is held on the substrate stably in terms of changes in length. Moreover, the scale should be held on the substrate at every point with the most constant possible contact pressure and in a manner free of drifting.